The viscosity of petroleum products generally varies drastically with temperature, that is, the viscosity has temperature dependency. When a lubricating oil is used for, for example, automobiles, the viscosity of the lubricating oil ideally has small temperature dependency. Therefore, to reduce the temperature dependency of the viscosity, lubricating oils contain a viscosity index improver, which is a specific polymer soluble in a lubricating oil base. Ethylene/α-olefin copolymers have been widely used in recent years as the viscosity index improver and modified in various ways to further improve the balance between properties of the lubricating oils.
The viscosity index improver is generally used to prevent a lowering of the lubricating oil viscosity at high temperatures. However in recent years, there has been a demand for a viscosity index improver that can suppress a rise of the lubricating oil viscosity at low temperatures (which means imparting excellent low temperature properties to the lubricating oil). Lubricating oil compositions are used for applications where a shear force is applied and therefore need to have a quality ensuring excellent shear stability. To improve the shear stability, a low-molecular weight polymer is generally used. However, as the molecular weight lowers, the viscosity index improver has to be added in a large amount to reach a viscosity that a lubricating oil needs to have, with the result that the economical efficiency is worsened. On the other hand, the shear stability is deteriorated when the viscosity index improver is added in a reduced amount with an increased molecular weight in an attempt to improve the economical efficiency of the lubricating oil.
The polymers used for lubricating oils are known to be better when they have a narrower molecular weight distribution. A representative advantage of such polymers is that the shear stability is good in comparison with other polymers having a broader molecular weight distribution. As such, various catalysts are now studied to narrow the molecular weight distribution of the polymers. Proposed catalysts include a catalyst supported on a highly active magnesium chloride, a metallocene catalyst and a vanadium catalyst. JP-A-60(1985)/35009 discloses a polymer having Mw/Mn of 1.2, which is obtained using these catalysts.
U.S. Pat. No. 3,697,429 discloses a blend of ethylene/α-olefin copolymers having different ethylene contents. It is described that a lubricating oil having suitable low temperature properties can be obtained when this blend is used as a viscosity index improver for lubricating oils. However, such a lubricating oil is insufficient in the balance of properties.
Meanwhile, other inventions have been made taking advantage of characteristics of the living polymerization. For example, JP-A-60(1985)/35009 discloses a random copolymer and a block copolymer each composed of ethylene and an α-olefin, in which the molecular weight distribution and the composition distribution are narrow and the composition differs in the molecule. It is also described that these copolymers are suitable as a viscosity index improver for lubricating oils because they have good shear stability and viscosity-increasing properties and excellent low temperature properties, which are essential to the applications of the lubricating oil. In particular, it is also mentioned that a block copolymer having segments of different compositions in the same molecule is more preferable. The use of a tubular reactor is disclosed as a method for continuously obtaining a polymer having a narrow molecular weight distribution in the presence of a catalyst enabling living polymerization. When a copolymer is produced in a tubular reactor, the monomer composition changes in the reactor because the monomers have different reactivity to yield a tapered copolymer in which the composition gradually differs in the polymer chain, giving a broad intramolecular composition distribution. Although the polymers disclosed in the above literature are suitable as a viscosity index improver for lubricating oils, the polymer described in detail in the same literature that has a molecular weight distribution (Mw/Mn) of as narrow as 1.2 does not have sufficient capabilities to improve low temperature properties and is requested for further improvement. As a method to impart the capabilities of improving low temperature properties, it is known to allow the polymer to contain ethylene in an amount as large as possible within the range not extremely detrimental to the low temperature properties. This method, however, has a problem that the intramolecular composition distribution in a tapered copolymer is so broadened that it is difficult to avoid such a detrimental range.
Furthermore, WO98/58978 (National Publication of International Patent 2002-507225) discloses a viscosity index improver for lubricating oils comprising an olefin block copolymer. One block of the above-disclosed olefin block copolymer comprises ethylene of about 93% by weight and this viscosity index improver for lubricating oils is still insufficient to improve low temperature properties.